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A new capillary gripper for mini and micro parts
CIRP Annals - Manufacturing Technology 62 (2013) 17–20
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CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
A new capillary gripper for mini and micro parts Gualtiero Fantoni a,*, Hans Nørgaard Hansen (1)b, Marco Santochi (1)a a b
Department of Civil and Industrial Engineering, University of Pisa, Largo Lucio Lazzarino, 2, 56125 Pisa, Italy Department of Mechanical Engineering, Technical University of Denmark, Produktionstorvet, Building 27A, Kgs. Lyngby 2800, Denmark
A R T I C L E I N F O
A B S T R A C T
Keywords: Assembly Handling Adhesive forces
In the assembly of microproducts the grasping and releasing phases are key tasks. Since in the microdomain gravity becomes negligible in comparison with adhesion forces, several reliable grasping methods have been developed. On the contrary, the releasing phase is still very critical because the part tends to stick to the gripper. In this paper a novel strategy based on capillary forces both for grasping and releasing is proposed. This novel grasping-releasing strategy exploits the transition between hydrophobic and hydrophilic surfaces to change the grasping force. The paper starts from the releasing problem in microassembly, deals with the manufacturing of hydrophobic and hydrophilic surfaces and demonstrates the use of such structures to grasp and release delicate mini and microparts. ß 2013 CIRP.
1. Introduction Driven by the silicon revolution microproducts emerged in the 1980 [1] and their growth is continuing in many fields: electronics, medical and biomedical, healthcare, automotive, biotechnology, telecommunications, optics, energy. Their world market value has reached a value of 57 billion dollar in 2009 [2]. Nevertheless their cost remains high owing to the assembly phase, not yet completely automated. That is especially true in case of hybrid microproducts where several components of different materials have to be assembled. Even if many research efforts have been devoted to automatize feeding, sorting, grasping and joining, micro parts remain very difficult to release. In fact, the presence of surface forces as van der Waals, electrostatic, etc. prevents their simple releasing through gravity [3] as it happens in standard assembly. Many principles have been exploited for micro grippers: from electrostatics to van der Waals, from capillary to ice grasping, from contactless handling as laser trapping to ultrasonic levitation or Bernoulli’s principle [3]. Among the developed grippers capillary forces have been used owing to their flexibility and reliability [1,4,5]. Their main features are: favourable downscaling law because capillary force is proportional to the linear dimension of components; compliant behaviour and a self-centring effect due to surface tension; capability of grasping microparts only through the top surface available; capability of grasping small and light components in a wide range of materials and shapes;
* Corresponding author. E-mail addresses: [email protected], [email protected], [email protected] (G. Fantoni). 0007-8506/$ – see front matter ß 2013 CIRP. http://dx.doi.org/10.1016/j.cirp.2013.03.005
capability of grasping delicate components as the meniscus between the gripper and the object has a ‘‘bumper’’ effect. However a reliable releasing of micro-objects is still a problem. Together with the development of microproducts, microengineered surfaces have been industrialized and have been spreading in everyday life [6]. As the surface texture reduces its size, different surface phenomena and functional properties can be observed and exploited. Optical phenomena as photonic effects or chemical effects as super-hydrophobicity or physical effects as gecko’s sticking toepads can be realized through micro and nano manufacturing techniques. For example by controlling surface roughness and its interlocking properties [7] it is possible to deeply affect surface adhesion properties. One of the widely known results of microtexturing allowed researchers to replicate the ‘‘lotus effect’’. Through a physical or chemical treatment, or by texturing the surfaces, industry started producing paints, roof tiles, fabrics and other (super) hydrophobic surfaces that can be kept always dry and clean. These so-called super-hydrophobic surfaces are based on three features: (i) hydrophobic materials, (ii) chemical properties, and (iii) a micrometric or sub-micrometric texture covering the surface. There, water drops remain almost spherical making liquid deposition practically impossible. In case of super-hydrophobicity water drops stand on the top of the surface asperities while air pockets remain trapped between the solid and the liquid surface. Therefore the liquid contacts only a fraction of the solid surface that supports the drops. The higher the hydrophobicity the higher the value of the contact angle between the liquid and the substrate. For ultra-rough substrates the contact angle is close to 1808 [8]. To produce such microstructured surfaces manufacturing techniques include material removal processes (e.g. laser machining), replication processes (e.g. hot embossing and injection moulding [9]), material deposition processes (e.g. PVD) as well as processes changing the chemical properties of the surface (e.g. corona treatment).
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G. Fantoni et al. / CIRP Annals - Manufacturing Technology 62 (2013) 17–20
2. Design of the hydrophobic/hydrophilic gripper 2.1. Theoretical analysis Fig. 1 shows a cross section of an axial-symmetric meniscus between a spherical part (r = R) and a flat gripper. The lifting force of an adhesive gripper is due to the liquid bridge between the gripper surface and the object surface. The liquid bridge has contact angles (b) and (a) with the gripper surface and the object surface respectively. The capillary force acting on an object (Eq. (1)) is the sum of Laplace and tension forces [4]. The Laplace force FL (Eq. (2)) is due to the pressure difference between a liquid and a gas and depends on the curvature of the interface. The tension force FT (Eq. (3)) is directly exerted by the vertical component of the surface tension. Fc ¼ FL þ FT
(1)
F L ¼ 2g HpR2a
(2)
F T ¼ 2pRa g sinðu þ aÞ
(3)
liquid bridge, but while electrowetting is based on electrically induced variation of the liquid contact angle b here a variation of the wetted surface at gripper level (xa reduction) is proposed. From a theoretical point of view if a hydrophobic ring (light grey parts in Fig. 1b and c) surrounds a droplet, the smaller is the radius of the internal circle (xa) the smaller the capillary lifting force (Fig. 1d–f). 2.2. The new principle of the gripper Fig. 2 illustrates the working principle of the capillary microgripper. The gripper consists of an elastomeric membrane, where a hydrophobic ring (light grey) surrounds a hydrophilic circle (dark grey). A mechanical structure is used to stretch the membrane and to change the dimension of the hydrophobic/ hydrophilic areas. The proposed micromanipulation consists of the following steps (Fig. 2).
In the previous equations g is the surface tension (for water g 72 103 N m1) and H equals 2(1/r0 1/r1). Ra is the radius of the circle formed between the object and the meniscus, u is the angle on the sphere at the interface with the liquid, xa is the radius of the contact line between the plane and the meniscus, r0 and r1 are the curvature radii of the meniscus. The numerical analysis of the above equations shows that: (i) all the variables depend on the volume of the liquid and on the distance h, and (ii) the grasping force has a maximum when h (the distance between the sphere and the gripper) is zero. Thus, if the object can be grasped (Fmax > mg, Fig. 1a–c), when h reaches the value of h* and the capillary force equals the weight (Fig. 1c), then it tends to be lifted and adheres to the gripper surface (Fig. 1d).
Fig. 1. (Centre) Geometry of a liquid meniscus between a flat gripper and a spherical micropart. (Left) shape of a meniscus during object grasping. Right) the shape of three menisci and related forces for different value of xa (dark grey: hydrophilic material; light grey hydrophobic material).
In adhesive microgrippers, the main releasing strategies [10] are the following: pushing microparts against an edge [4]; modifying the curvature of the gripper (from flat to rounded, to sharp tip) [5]; using air pressure by an injection of gas [11]; squeezing the meniscus through electrowetting [12]. The new conceived principle for releasing the grasped microparts acts similarly to electrowetting. Both squeeze the
Fig. 2. Preparation (a–c), grasping (d–e), handling (f) and releasing (g–h) process.
The gripper approaches the water basin with the membrane in the relaxed condition (a). The hydrophobic ring limits the quantity of water that sticks to the hydrophilic inner circular surface (b) and so a droplet is captured (c). The membrane is stretched radially, then the gripper approaches the object (d). When the contact between the water surface occurs the object is picked up through the capillary force (e and f). Once the object is grasped and placed in the final position it can be released by relaxing the membrane. The hydrophobic ring starts reducing its size and squeezing the droplet until it breaks (g). Thus the object is released (h). The water droplet remains in part on the gripper and in part on the object (h). Then the process starts again (a0 ). The lifting force has a maximum (Fmax) when the strain on the membrane is the highest and xa reaches its maximum, conversely it has a minimum when the membrane is relaxed and xa is minimum. Thus an object with weight between Fmax and Fmin can be picked up and released. The development of the new gripper had two main problems: the choice of the material and the manufacturing process. Actually the material must: (i) be elastic, (ii) have hydrophilic/hydrophilic behavior and (iii) maintain such surface properties during the stretching. Moreover (iv) the dimension of the inner circle has to be small (<1 mm). Therefore a deep analysis of the manufacturing process and of the obtained properties has been necessary. 3. The influence of the manufacturing process From the manufacturing point of view the gripper requires a hydrophobic ring (D = 3 mm) and a hydrophilic circle (d = 0.5 mm) manufactured on a deformable membrane (Fig. 2c). One of the candidate processes was laser induced selective activation (LISA) developed as a new technique for selective metallization of a polymer surface. The polymer surface is modified
G. Fantoni et al. / CIRP Annals - Manufacturing Technology 62 (2013) 17–20
by laser in a medium of deionized water, so the surface becomes porous and rough. This property of the surface favours the chemical activation and subsequent metallisation of the polymer [13]. Only the laser part of the process chain is applied in this work. The polymeric material is heated by the laser beam, it boils and the generated bubbles are immediately ‘‘frozen’’ by the water. After the surface has been dried, the porous surface has become hydrophobic as described in [8]. A black grade rubber membrane was chosen as material and LISA as process. A lamp-pumped Q-switch Nd:YAG laser (l = 1064 mm) was used in the process. The rubber surface was modified as described above. The laser beam followed a cycloid path to increase the machined area. The beam feed optimized for LISA was 60 mm/s and the average output power was 3.4 W. The hypothesis to be verified through the preliminary tests was the possibility to extend the LISA process also to black grade elastic rubbers. Preliminary results demonstrated a change in wettability of the surface but the physics of the process appeared to be different. Actually, after the LISA process, the water contained black particles of the same composition as the rubber. Therefore, the laser burns the carbon black in the rubber or acts on the vulcanized rubber breaking some of the atomic bonds among sulphur or carbon atoms to carbon atoms. Thus the process creates the rough hydrophobic surface by engraving. Since the gripper’s membrane varies its size during the grasping and releasing phases, the texture of the surface at micro-scale changes during stretching. Therefore it was necessary to understand if and how the hydrophobic properties of the surface change during the deformation of the membrane. Moreover, the membrane can be engraved in different conditions of stretching. Therefore it is interesting to understand in which conditions the engraving process produces the best results (i.e. the higher difference in hydrophobicity between engraved and not-engraved surfaces). Therefore the engraving conditions and the working conditions were investigated by means of two factors – three levels full factorial design of experiments (DOE) for a total number of 32 = 9 experiments. Table 1 shows the DOE. The experiments have been designed in the following way: an area of 9 mm 8 mm in the rubber membrane (black grade) has been engraved by LISA. The pattern engraved by the laser consisted of a series of 50 parallel lines, each line had a cycloid pattern generating an apparent line thickness of 0.4 mm. The membrane surface was engraved (E) in the three following conditions: relaxed (ER), stretched (ES), over stretched (EO). The strain of the rubber (e = (lS lR) 100/lR) was eR = 0%, eS = 23% and eSS = 38%, respectively. Three samples for each stretching condition were manufactured. For each condition the sample was measured in three working (W) conditions: relaxed (WR), stretched (WS) and over stretched conditions (WO).
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Fig. 3a shows a scanning electron microscope image of the standard rubber and of the rubber engraved under over stretching conditions and measured in relaxed conditions. The SEM images show a flat surface of standard rubber and a very rough geometry when engraved. Fig. 3b compares two images taken via optical microscope and shows a remarkable presence of air bubbles trapped between the rubber surface and the water droplet. The texture of the machined surface consists of pillars and valleys which provide the ‘‘fakir’’ effect [8] and a very high level of hydrophobicity. Fig. 3c compares two images concerning the wetting conditions of standard and machined surfaces as obtained through the CCD camera mounted in the optical contact angle measuring device. The liquid used for the measurement was distilled water and the room temperature 25 8C. Contact angles under the different
Fig. 3. Comparison between standard and engraved surfaces in (a) SEM images, (b) optical microscope pictures, and (c) optical contact angle images.
stretching conditions have been measured. The measurements have been repeated 8 times each sample and the lowest and highest values have been discarded. The measurement of the static angle in case of highly hydrophobic surfaces is not trivial for many reasons: (i) the dispensed droplet tends to be repelled by the surface and to stick to the needle; (ii) the air bubbles inside the droplet tend to collapse in time therefore the angle varies second after second; (iii) the shape of the droplet is not symmetric; (iv) the software underestimates the contact angle values when the surface is very hydrophobic (semiautomatic analysis supplies values about 58 greater than automatic one). For these reasons the higher values reported in the ANOVA are probably underestimated. As shown in the box plot diagram (Fig. 4) data for each sample are usually dispersed. This is probably due to dust particles on the surface and to different positions of the droplet over the surface.
Table 1 Design of experiments.
Engraving condion s
E R /… E S /… E O/…
…/ W R E R /W R E S /W R E o/W R
Working condions …/ W S E R /W S E S /W S E o/W S
…/ W O E R /W O E S /W O E o/W O
4. Experimental results on machined surfaces The measurement of the contact angles is obtained through the standard method to determine the hydrophobic properties of a surface. It was performed by an optical contact angle measuring machine and its static contact angle software. The measurement was obtained by a CCD camera recording the dispensing or the suction of a droplet through a cylindrical needle (it can be observed in Fig. 3c). The measurement was static and the contact angle b was measured while dispensing the droplet.
Fig. 4. ANOVA of contact angles.
Fig. 4 shows the results of the ANOVA and the related box plot. In the yellow box the contact angle of distilled water on standard rubber. The next three columns represent the contact angles measured on relaxed specimens (../WR), engraved surface in case of
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G. Fantoni et al. / CIRP Annals - Manufacturing Technology 62 (2013) 17–20
no stress (ER/..), stretched (ES/..), and over stretched (EO/..) conditions. The three measurements show an increasing value of contact angles. That means that the higher the stretching of the surface during the engraving, the higher the hydrophobicity. The same behaviour can be observed on the following two triplets: in stretched working conditions ../WS and in over stretched working conditions ../WO, respectively (blue arrows in Fig. 4 and in Table 1). Moreover maintaining the same initial conditions of stress during the engraving (e.g. Es/..) the contact angles increase with the stretching of the sample in the working conditions. The variation in contact angles is greater between relaxed working conditions ../WR and stretched ../WS or over stretched working conditions ../WO. Conversely, almost no difference can be observed between stretched ../WS and super stretched working conditions ../WO (red arrows in Fig. 4 and in Table 1). The conclusion is that the higher the deformation of the membrane during the engraving and during the working phase, the higher the value of hydrophobicity that can be achieved. Therefore a ring with 0.5 mm internal diameter and 3 mm as external diameter was engraved in stretched conditions. Actually, the hydrophobicity is high and the stress on the membrane is low.
5. Grasping and releasing experiments The LISA process on the rubber membrane produced two surfaces with very different wetting condition (from b = 1208 to b = 1608). They are not rigorously hydrophilic/hydrophobic conditions but the different values of the contact angles can generate the squeezing effect on the droplet. Therefore the aim of the grasping–releasing experiments was to determine the limits of the designed gripper and which components can be properly handled. An experimental set up was built (see Fig. 5). The expanding mechanism is composed of a polypropylene cylinder divided into four sectors and a conical tip that, moving up and down into the cylinder, deforms the four sectors. The rubber membrane is glued on the polypropylene cylinder and follows the deformation induced on the four sectors by the conical tip. Thus, the membrane is stretched. The membrane had a ring in the middle (internal diameter = 0.5 mm) and demineralised water was used as grasping liquid. The process described in Fig. 2 was followed during grasping and releasing of the tested components.
conducted in an environment without restrictions on cleanness and humidity: temperature was about 25 8C and relative humidity about 40%. Fig. 5 shows the results of both grasping and releasing. The experiments demonstrated that the manufactured gripper is able to grasp and release both flat and spherical components in a certain range of dimensions and weights in standard industrial conditions. When the mass is low the object is grasped but its releasing is impossible or not reliable, conversely when the mass is higher the object grasping becomes difficult thus reducing the grasping–releasing reliability. Spheres and flat screws with the same weight but different contact surfaces shows different behaviours because of the different meniscus conditions and contact angles of metal and glass. 6. Conclusion In this paper a new capillary microgripper for grasping and releasing mini and microparts has been designed, manufactured and successfully tested. Furthermore a recent manufacturing process (LISA) has been tested on an elastomeric rubber black grade with good performance. The developed microgripper demonstrated good capability in grasping and releasing parts of different size, shape and material. Its main drawbacks are the following: (i) dust particles that stick to the water droplet as in other capillary grippers in literature (ii) the LISA process alters the rubber. In fact the reliability of the gripper reduces because of small cracks which propagate on the membrane after a few hundreds opening-closing cycles.
In the future the analysis of new materials and other manufacturing experiments will be oriented to overcome the membrane degradation. Acknowledgments The authors would like to thank Y. Zhang, PhD for the manufacturing of the rubber membranes and Mr. Tincani and Mr. Barbuti for the design of the microassembly set up. This work was partially supported by RobLog Project (FP7 ICT-270350). References
Fig. 5. Experimental set up and tested components.
The tested components were spheres whose diameter varies between d = 0.50–2.85 mm and weight between 3–373 mN and flat components as metal mini-screws whose head diameter varies between D = 0.5–2.5 mm and weight between 28–310 mN. To simulate the standard industrial conditions, experiments were
[1] Alting L, Kimura FN, Hansen H, Bissacco G, Micro Engineering (2003) Annals of the CIRP 52(2):635–657. [2] NEXUS Market Analysis for MEMS and Microsystems III, 2005–2009. [3] Van Brussel H, et al (2000) Assembly of Microsystem. Annals of the CIRP 49(2):451–472. [4] Lambert P (2007) Capillary Forces in Microassembly: Modeling, Simulation, Experiments, and Case Study. Microtechnology and MEMS, Springer, New York. [5] Biganzoli F, Fassi I, Pagano C (2005) Development of a Gripping System Based on a Capillary Force. Proceedings of IEEE ISATP 36–40. [6] Bruzzone AAG, Costa HL, Lonardo PM, Lucca DA (2008) Advances in Engineered Surfaces for Functional Performance. CIRP Annals – Manufacturing Technology 57(2):750–769. [7] Bico J, Thiele U, Quere D (2002) Wetting of Textured Surfaces. Colloids and Surfaces A 206(1–3):41–46. [8] Moulinet S, Bartolo D (2007) Life and Death of a Fakir Droplet: Impalement Transitions on Superhydrophobic Surfaces. European Physical Journal E 24:251–260. [9] Hansen HN, Hocken RJ, Tosello G (2011) Replication of Micro and Nano Surface Geometries. CIRP Annals 60(2):695–714. [10] Fantoni G, Porta M (2008) A Critical Review of Releasing Strategies in Microparts Handling. Fourth International Precision Assembly Seminar IPAS’2008, Chamonix 11–13 February. [11] Bark C, Binneboese T (1998) Gripping with Low Viscosity Fluid. IEEE International Workshop on MEMS 301–305. [12] Vasudev A, Zhe J (2009) A Low Voltage Capillary Microgripper Using Electrowetting. TRANSDUCERS 2009 – 15th International Conference on Solid-State Sensors Actuators and Microsystems, Denver, COpp. 825–828. [13] Zhang Y, Hansen HN, De grave A, Tang PT, Nielsen JS (2011) Selective Metallization of Polymers Using Laser Induced Surface Activation (LISA) – Characterization and Optimization of Porous Surface Topography. International Journal of Advanced Manufacturing Technology 55(5–8):573–580.
Contents lists available at SciVerse ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
A new capillary gripper for mini and micro parts Gualtiero Fantoni a,*, Hans Nørgaard Hansen (1)b, Marco Santochi (1)a a b
Department of Civil and Industrial Engineering, University of Pisa, Largo Lucio Lazzarino, 2, 56125 Pisa, Italy Department of Mechanical Engineering, Technical University of Denmark, Produktionstorvet, Building 27A, Kgs. Lyngby 2800, Denmark
A R T I C L E I N F O
A B S T R A C T
Keywords: Assembly Handling Adhesive forces
In the assembly of microproducts the grasping and releasing phases are key tasks. Since in the microdomain gravity becomes negligible in comparison with adhesion forces, several reliable grasping methods have been developed. On the contrary, the releasing phase is still very critical because the part tends to stick to the gripper. In this paper a novel strategy based on capillary forces both for grasping and releasing is proposed. This novel grasping-releasing strategy exploits the transition between hydrophobic and hydrophilic surfaces to change the grasping force. The paper starts from the releasing problem in microassembly, deals with the manufacturing of hydrophobic and hydrophilic surfaces and demonstrates the use of such structures to grasp and release delicate mini and microparts. ß 2013 CIRP.
1. Introduction Driven by the silicon revolution microproducts emerged in the 1980 [1] and their growth is continuing in many fields: electronics, medical and biomedical, healthcare, automotive, biotechnology, telecommunications, optics, energy. Their world market value has reached a value of 57 billion dollar in 2009 [2]. Nevertheless their cost remains high owing to the assembly phase, not yet completely automated. That is especially true in case of hybrid microproducts where several components of different materials have to be assembled. Even if many research efforts have been devoted to automatize feeding, sorting, grasping and joining, micro parts remain very difficult to release. In fact, the presence of surface forces as van der Waals, electrostatic, etc. prevents their simple releasing through gravity [3] as it happens in standard assembly. Many principles have been exploited for micro grippers: from electrostatics to van der Waals, from capillary to ice grasping, from contactless handling as laser trapping to ultrasonic levitation or Bernoulli’s principle [3]. Among the developed grippers capillary forces have been used owing to their flexibility and reliability [1,4,5]. Their main features are: favourable downscaling law because capillary force is proportional to the linear dimension of components; compliant behaviour and a self-centring effect due to surface tension; capability of grasping microparts only through the top surface available; capability of grasping small and light components in a wide range of materials and shapes;
* Corresponding author. E-mail addresses: [email protected], [email protected], [email protected] (G. Fantoni). 0007-8506/$ – see front matter ß 2013 CIRP. http://dx.doi.org/10.1016/j.cirp.2013.03.005
capability of grasping delicate components as the meniscus between the gripper and the object has a ‘‘bumper’’ effect. However a reliable releasing of micro-objects is still a problem. Together with the development of microproducts, microengineered surfaces have been industrialized and have been spreading in everyday life [6]. As the surface texture reduces its size, different surface phenomena and functional properties can be observed and exploited. Optical phenomena as photonic effects or chemical effects as super-hydrophobicity or physical effects as gecko’s sticking toepads can be realized through micro and nano manufacturing techniques. For example by controlling surface roughness and its interlocking properties [7] it is possible to deeply affect surface adhesion properties. One of the widely known results of microtexturing allowed researchers to replicate the ‘‘lotus effect’’. Through a physical or chemical treatment, or by texturing the surfaces, industry started producing paints, roof tiles, fabrics and other (super) hydrophobic surfaces that can be kept always dry and clean. These so-called super-hydrophobic surfaces are based on three features: (i) hydrophobic materials, (ii) chemical properties, and (iii) a micrometric or sub-micrometric texture covering the surface. There, water drops remain almost spherical making liquid deposition practically impossible. In case of super-hydrophobicity water drops stand on the top of the surface asperities while air pockets remain trapped between the solid and the liquid surface. Therefore the liquid contacts only a fraction of the solid surface that supports the drops. The higher the hydrophobicity the higher the value of the contact angle between the liquid and the substrate. For ultra-rough substrates the contact angle is close to 1808 [8]. To produce such microstructured surfaces manufacturing techniques include material removal processes (e.g. laser machining), replication processes (e.g. hot embossing and injection moulding [9]), material deposition processes (e.g. PVD) as well as processes changing the chemical properties of the surface (e.g. corona treatment).
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G. Fantoni et al. / CIRP Annals - Manufacturing Technology 62 (2013) 17–20
2. Design of the hydrophobic/hydrophilic gripper 2.1. Theoretical analysis Fig. 1 shows a cross section of an axial-symmetric meniscus between a spherical part (r = R) and a flat gripper. The lifting force of an adhesive gripper is due to the liquid bridge between the gripper surface and the object surface. The liquid bridge has contact angles (b) and (a) with the gripper surface and the object surface respectively. The capillary force acting on an object (Eq. (1)) is the sum of Laplace and tension forces [4]. The Laplace force FL (Eq. (2)) is due to the pressure difference between a liquid and a gas and depends on the curvature of the interface. The tension force FT (Eq. (3)) is directly exerted by the vertical component of the surface tension. Fc ¼ FL þ FT
(1)
F L ¼ 2g HpR2a
(2)
F T ¼ 2pRa g sinðu þ aÞ
(3)
liquid bridge, but while electrowetting is based on electrically induced variation of the liquid contact angle b here a variation of the wetted surface at gripper level (xa reduction) is proposed. From a theoretical point of view if a hydrophobic ring (light grey parts in Fig. 1b and c) surrounds a droplet, the smaller is the radius of the internal circle (xa) the smaller the capillary lifting force (Fig. 1d–f). 2.2. The new principle of the gripper Fig. 2 illustrates the working principle of the capillary microgripper. The gripper consists of an elastomeric membrane, where a hydrophobic ring (light grey) surrounds a hydrophilic circle (dark grey). A mechanical structure is used to stretch the membrane and to change the dimension of the hydrophobic/ hydrophilic areas. The proposed micromanipulation consists of the following steps (Fig. 2).
In the previous equations g is the surface tension (for water g 72 103 N m1) and H equals 2(1/r0 1/r1). Ra is the radius of the circle formed between the object and the meniscus, u is the angle on the sphere at the interface with the liquid, xa is the radius of the contact line between the plane and the meniscus, r0 and r1 are the curvature radii of the meniscus. The numerical analysis of the above equations shows that: (i) all the variables depend on the volume of the liquid and on the distance h, and (ii) the grasping force has a maximum when h (the distance between the sphere and the gripper) is zero. Thus, if the object can be grasped (Fmax > mg, Fig. 1a–c), when h reaches the value of h* and the capillary force equals the weight (Fig. 1c), then it tends to be lifted and adheres to the gripper surface (Fig. 1d).
Fig. 1. (Centre) Geometry of a liquid meniscus between a flat gripper and a spherical micropart. (Left) shape of a meniscus during object grasping. Right) the shape of three menisci and related forces for different value of xa (dark grey: hydrophilic material; light grey hydrophobic material).
In adhesive microgrippers, the main releasing strategies [10] are the following: pushing microparts against an edge [4]; modifying the curvature of the gripper (from flat to rounded, to sharp tip) [5]; using air pressure by an injection of gas [11]; squeezing the meniscus through electrowetting [12]. The new conceived principle for releasing the grasped microparts acts similarly to electrowetting. Both squeeze the
Fig. 2. Preparation (a–c), grasping (d–e), handling (f) and releasing (g–h) process.
The gripper approaches the water basin with the membrane in the relaxed condition (a). The hydrophobic ring limits the quantity of water that sticks to the hydrophilic inner circular surface (b) and so a droplet is captured (c). The membrane is stretched radially, then the gripper approaches the object (d). When the contact between the water surface occurs the object is picked up through the capillary force (e and f). Once the object is grasped and placed in the final position it can be released by relaxing the membrane. The hydrophobic ring starts reducing its size and squeezing the droplet until it breaks (g). Thus the object is released (h). The water droplet remains in part on the gripper and in part on the object (h). Then the process starts again (a0 ). The lifting force has a maximum (Fmax) when the strain on the membrane is the highest and xa reaches its maximum, conversely it has a minimum when the membrane is relaxed and xa is minimum. Thus an object with weight between Fmax and Fmin can be picked up and released. The development of the new gripper had two main problems: the choice of the material and the manufacturing process. Actually the material must: (i) be elastic, (ii) have hydrophilic/hydrophilic behavior and (iii) maintain such surface properties during the stretching. Moreover (iv) the dimension of the inner circle has to be small (<1 mm). Therefore a deep analysis of the manufacturing process and of the obtained properties has been necessary. 3. The influence of the manufacturing process From the manufacturing point of view the gripper requires a hydrophobic ring (D = 3 mm) and a hydrophilic circle (d = 0.5 mm) manufactured on a deformable membrane (Fig. 2c). One of the candidate processes was laser induced selective activation (LISA) developed as a new technique for selective metallization of a polymer surface. The polymer surface is modified
G. Fantoni et al. / CIRP Annals - Manufacturing Technology 62 (2013) 17–20
by laser in a medium of deionized water, so the surface becomes porous and rough. This property of the surface favours the chemical activation and subsequent metallisation of the polymer [13]. Only the laser part of the process chain is applied in this work. The polymeric material is heated by the laser beam, it boils and the generated bubbles are immediately ‘‘frozen’’ by the water. After the surface has been dried, the porous surface has become hydrophobic as described in [8]. A black grade rubber membrane was chosen as material and LISA as process. A lamp-pumped Q-switch Nd:YAG laser (l = 1064 mm) was used in the process. The rubber surface was modified as described above. The laser beam followed a cycloid path to increase the machined area. The beam feed optimized for LISA was 60 mm/s and the average output power was 3.4 W. The hypothesis to be verified through the preliminary tests was the possibility to extend the LISA process also to black grade elastic rubbers. Preliminary results demonstrated a change in wettability of the surface but the physics of the process appeared to be different. Actually, after the LISA process, the water contained black particles of the same composition as the rubber. Therefore, the laser burns the carbon black in the rubber or acts on the vulcanized rubber breaking some of the atomic bonds among sulphur or carbon atoms to carbon atoms. Thus the process creates the rough hydrophobic surface by engraving. Since the gripper’s membrane varies its size during the grasping and releasing phases, the texture of the surface at micro-scale changes during stretching. Therefore it was necessary to understand if and how the hydrophobic properties of the surface change during the deformation of the membrane. Moreover, the membrane can be engraved in different conditions of stretching. Therefore it is interesting to understand in which conditions the engraving process produces the best results (i.e. the higher difference in hydrophobicity between engraved and not-engraved surfaces). Therefore the engraving conditions and the working conditions were investigated by means of two factors – three levels full factorial design of experiments (DOE) for a total number of 32 = 9 experiments. Table 1 shows the DOE. The experiments have been designed in the following way: an area of 9 mm 8 mm in the rubber membrane (black grade) has been engraved by LISA. The pattern engraved by the laser consisted of a series of 50 parallel lines, each line had a cycloid pattern generating an apparent line thickness of 0.4 mm. The membrane surface was engraved (E) in the three following conditions: relaxed (ER), stretched (ES), over stretched (EO). The strain of the rubber (e = (lS lR) 100/lR) was eR = 0%, eS = 23% and eSS = 38%, respectively. Three samples for each stretching condition were manufactured. For each condition the sample was measured in three working (W) conditions: relaxed (WR), stretched (WS) and over stretched conditions (WO).
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Fig. 3a shows a scanning electron microscope image of the standard rubber and of the rubber engraved under over stretching conditions and measured in relaxed conditions. The SEM images show a flat surface of standard rubber and a very rough geometry when engraved. Fig. 3b compares two images taken via optical microscope and shows a remarkable presence of air bubbles trapped between the rubber surface and the water droplet. The texture of the machined surface consists of pillars and valleys which provide the ‘‘fakir’’ effect [8] and a very high level of hydrophobicity. Fig. 3c compares two images concerning the wetting conditions of standard and machined surfaces as obtained through the CCD camera mounted in the optical contact angle measuring device. The liquid used for the measurement was distilled water and the room temperature 25 8C. Contact angles under the different
Fig. 3. Comparison between standard and engraved surfaces in (a) SEM images, (b) optical microscope pictures, and (c) optical contact angle images.
stretching conditions have been measured. The measurements have been repeated 8 times each sample and the lowest and highest values have been discarded. The measurement of the static angle in case of highly hydrophobic surfaces is not trivial for many reasons: (i) the dispensed droplet tends to be repelled by the surface and to stick to the needle; (ii) the air bubbles inside the droplet tend to collapse in time therefore the angle varies second after second; (iii) the shape of the droplet is not symmetric; (iv) the software underestimates the contact angle values when the surface is very hydrophobic (semiautomatic analysis supplies values about 58 greater than automatic one). For these reasons the higher values reported in the ANOVA are probably underestimated. As shown in the box plot diagram (Fig. 4) data for each sample are usually dispersed. This is probably due to dust particles on the surface and to different positions of the droplet over the surface.
Table 1 Design of experiments.
Engraving condion s
E R /… E S /… E O/…
…/ W R E R /W R E S /W R E o/W R
Working condions …/ W S E R /W S E S /W S E o/W S
…/ W O E R /W O E S /W O E o/W O
4. Experimental results on machined surfaces The measurement of the contact angles is obtained through the standard method to determine the hydrophobic properties of a surface. It was performed by an optical contact angle measuring machine and its static contact angle software. The measurement was obtained by a CCD camera recording the dispensing or the suction of a droplet through a cylindrical needle (it can be observed in Fig. 3c). The measurement was static and the contact angle b was measured while dispensing the droplet.
Fig. 4. ANOVA of contact angles.
Fig. 4 shows the results of the ANOVA and the related box plot. In the yellow box the contact angle of distilled water on standard rubber. The next three columns represent the contact angles measured on relaxed specimens (../WR), engraved surface in case of
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G. Fantoni et al. / CIRP Annals - Manufacturing Technology 62 (2013) 17–20
no stress (ER/..), stretched (ES/..), and over stretched (EO/..) conditions. The three measurements show an increasing value of contact angles. That means that the higher the stretching of the surface during the engraving, the higher the hydrophobicity. The same behaviour can be observed on the following two triplets: in stretched working conditions ../WS and in over stretched working conditions ../WO, respectively (blue arrows in Fig. 4 and in Table 1). Moreover maintaining the same initial conditions of stress during the engraving (e.g. Es/..) the contact angles increase with the stretching of the sample in the working conditions. The variation in contact angles is greater between relaxed working conditions ../WR and stretched ../WS or over stretched working conditions ../WO. Conversely, almost no difference can be observed between stretched ../WS and super stretched working conditions ../WO (red arrows in Fig. 4 and in Table 1). The conclusion is that the higher the deformation of the membrane during the engraving and during the working phase, the higher the value of hydrophobicity that can be achieved. Therefore a ring with 0.5 mm internal diameter and 3 mm as external diameter was engraved in stretched conditions. Actually, the hydrophobicity is high and the stress on the membrane is low.
5. Grasping and releasing experiments The LISA process on the rubber membrane produced two surfaces with very different wetting condition (from b = 1208 to b = 1608). They are not rigorously hydrophilic/hydrophobic conditions but the different values of the contact angles can generate the squeezing effect on the droplet. Therefore the aim of the grasping–releasing experiments was to determine the limits of the designed gripper and which components can be properly handled. An experimental set up was built (see Fig. 5). The expanding mechanism is composed of a polypropylene cylinder divided into four sectors and a conical tip that, moving up and down into the cylinder, deforms the four sectors. The rubber membrane is glued on the polypropylene cylinder and follows the deformation induced on the four sectors by the conical tip. Thus, the membrane is stretched. The membrane had a ring in the middle (internal diameter = 0.5 mm) and demineralised water was used as grasping liquid. The process described in Fig. 2 was followed during grasping and releasing of the tested components.
conducted in an environment without restrictions on cleanness and humidity: temperature was about 25 8C and relative humidity about 40%. Fig. 5 shows the results of both grasping and releasing. The experiments demonstrated that the manufactured gripper is able to grasp and release both flat and spherical components in a certain range of dimensions and weights in standard industrial conditions. When the mass is low the object is grasped but its releasing is impossible or not reliable, conversely when the mass is higher the object grasping becomes difficult thus reducing the grasping–releasing reliability. Spheres and flat screws with the same weight but different contact surfaces shows different behaviours because of the different meniscus conditions and contact angles of metal and glass. 6. Conclusion In this paper a new capillary microgripper for grasping and releasing mini and microparts has been designed, manufactured and successfully tested. Furthermore a recent manufacturing process (LISA) has been tested on an elastomeric rubber black grade with good performance. The developed microgripper demonstrated good capability in grasping and releasing parts of different size, shape and material. Its main drawbacks are the following: (i) dust particles that stick to the water droplet as in other capillary grippers in literature (ii) the LISA process alters the rubber. In fact the reliability of the gripper reduces because of small cracks which propagate on the membrane after a few hundreds opening-closing cycles.
In the future the analysis of new materials and other manufacturing experiments will be oriented to overcome the membrane degradation. Acknowledgments The authors would like to thank Y. Zhang, PhD for the manufacturing of the rubber membranes and Mr. Tincani and Mr. Barbuti for the design of the microassembly set up. This work was partially supported by RobLog Project (FP7 ICT-270350). References
Fig. 5. Experimental set up and tested components.
The tested components were spheres whose diameter varies between d = 0.50–2.85 mm and weight between 3–373 mN and flat components as metal mini-screws whose head diameter varies between D = 0.5–2.5 mm and weight between 28–310 mN. To simulate the standard industrial conditions, experiments were
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